EP0404624B1 - Method for controlling pressurized screening device and pressurized screening device - Google Patents

Abstract

The invention relates to a method for controlling pressurized screening devices. Fibre pulp is fed to the pressurized screening device and classified by means of a perforated screenplate (7) into the accepted portion of the pulp passing through perforations of said screenplate (7) and the rejected portion which is removed from the screenplate. The perforated screenplate (7) is treated by means of a device (11) moving relative to it. The screenplate (7) is cylindrical and the device (11) for treating it and moving relative to it is designed to rotate round the centre line of the cross-section of the cylindrical form of the screenplate (7). Fibre pulp is fed essentially parallel with the centre line of the screenplate to a plurality of baffle blades (12) comprising two or more baffle blades (13). The plurality of baffle blades (12) is designed to rotate essentially round said centre. The speed of rotation of the plurality of baffle blades and/or the radial dimension of the baffle blades (13) of the plurality of baffle blades (12) are controlled. The invention additionally relates to pressurized screening devices for implementing the method.

Description

• The invention relates to a method for controlling pressurized screening devices wherein fibre pulp is fed into the pressurized screening device and classified by means of a perforated screenplate into an accepted portion passing through said perforations of the screenplate and into a rejected portion which is removed from the screenplate, wherein the perforated screenplate is treated by means of a device moving relative to it, wherein the screenplate is cylindrical and the device for treating it and moving relative to it is designed to be rotated round the centre line of the cross-section of the cylindrical form of the screenplate.
• Several different designs for pressurized screening devices of this kind are known, for instance, in patent publications. As to prior art reference can be made to patent publications FI-A-56415 and FI-A-70059.
• In screening, the purity of the accepted pulp and the capacity of the pressurized screening device are affected by the screenplate and by.the device which moves relative to it and treats it. Perforation size in the screenplate has influence both on the capacity and the purity of the accepted portion. Also the surface profile e.g. profiled or smooth, has influence on the same quantities. The function of the device treating the screenplate, often given the technical term " rotor ", is to keep the screenplate clean, i.e. to break the fibre mat formations on its surface.
• Experiments have shown that, in screening devices, the tangential velocity of the fibre pulp to be screened has a major effect on the capacity and the purity of the accepted fibre pulp. Experiments have revealed that, if the tangential velocity of the fibre pulp in the pressurized screening device approaches zero, the capacity of the pressurized screener will be high and the screening result will be poor. If, on the other hand, the tangential velocity of the fibre pulp in the pressurized screening device corresponds closely to that of the device (rotor) treating the screenplate, the capacity will become low and the screening result will become better.
• The most common design of prior art is such where the fibre pulp in the screening device rotates much slower than the device (rotor) treating the screenplate. The situation becomes even worse in such pressurized screening devices where the tangential velocity of the fibre pulp varies in different parts of the screenplate; this effect becomes particularly crucial in the vertical direction of the screenplate in most designs of the pressurized screening devices presently in use. Therefore, the flow dynamics inside the pressurized screening device is unknown and indefinite. The situation in pressurized screening devices according to prior art is such that the tangential velocity of the fibre pulp is under control only in such pressurized screening devices where the pulp is forced to rotate along with the device (rotor) treating the screenplate.
• The method and the pressurized screening device exploiting the method aim at the realization of a controlled tangential velocity of the fibre pulp relative to the velocity of the devices treating the screenplate, particularly relative to the velocity of the rotor scrapers. The application of the method of this invention enables one always to choose the most favorable capacity/quality ratio of screening for the process. In other words, the method according to the invention enables one to control the separating efficiency and/or the capacity in an optimal way necessary for the process.
• To accomplish the above objects, a first aspect of the invention concerns a method of controlling a pressurised screening device wherein fibre pulp is fed into the pressurised screening device and classified by means of a cylindrically shaped perforated screenplate into an accepted portion passing through said perforations of the screenplate and into a rejected portion which is removed from the screenplate, wherein the perforated screenplate is treated by means of a screenplate treating device located within and moving relative to said screenplate, wherein the screenplate treating device is designed to be rotated round the centre-line of the cross-section of the cylindrical form of the screenplate, characterized in that fibre pulp is fed essentially parallel with the cylindrical axis centre line of the screenplate into a baffle blade assembly located within said screenplate and comprising two or more baffle blades each having an outer edge, i.e. the edge that faces the screenplate, essentially parallel with said screenplate; the baffle blade assembly is caused to rotate essentially round said cylindrical axis; and in that the speed of rotation of said baffle blade assembly and/or the radial position of the outer edge of said baffle blades is controlled.
• The technical procedure described above provides a wide-ranging control action which enables one to control within wide limits the tangential velocity of the fibre pulp leaving the plurality of baffle blades.
• A second aspect of the invention concerns a device comprising a cylindrically shaped perforated screenplate placed inside a housing, a screenplate treating device located within and moveable relative to said screenplate, means for generating a relative motion between the screenplate and said screenplate treating device, and piping means firstly for introducing fibre pulp into the pressurized screening device, secondly for removing the accepted portion of fiber pulp from the pressurized screening device and thirdly for removing the rejected portion of fiber pulp from the pressurized screening device characterized in that said pressurized screening device further comprises a baffle blade assembly located within said screenplate and comprised of two or more baffle blades each having an outer edge, i.e. the edge that faces the screenplate, essentially parallel with said screenplate, said baffle blade assemply being rotational essentially round an axis parallel with the centre line of the screenplate; said piping means feed the pulp into said pressurized screening device essentially parallel with the centre-line of the screenplate towards the plurality of baffle blades.
• The enclosed subclaims present advantageous embodiments of the pressurized screening device.
• The invention is further illustrated in the following description wherein reference is made to the enclosed drawings in which figures 1 and 2 show, as partially vertical cross-sections, some embodiments of pressurized screening devices exploiting the method according to the invention.
• The pressurized screening devices of figures 1 and 2 in the drawings are mounted on bed A and they comprise bottom 1 having an uprightly directed cylindrical housing 2 with a cover 3 in the upper part. The cover includes a feed connection unit 4 in the centre of the cylinder. The outer part of housing 2 is provided with an annular space 5 for the accepted portion, the space extending essentially along the height of the housing, the annular space having a connection unit 6 for removing the accepted portion out of the pressurized screening device to the next process stage. The inner wall of the annular space 5 is formed by the cylindrical screenplate 7 provided with perforations. The shaft assembly 8 mounted on bearings 9 on bed 1 and designed to rotate round the centre line of the pressurized screening device goes through bed 1. The shaft assembly 8 rotates round the centre line of screenplate 7 and a closed supporting structure 10, being at least partly conical, is attached to its end, the supporting structure enlarging from shaft assembly 8 starting from the narrow end, which is directed to the feed connection unit, towards bed 1. Elements 11, which treat screenplate 7 and which comprise of elongated form parts, are attached to the wider edge of the conical supporting structure 10 at specific intervals on its circumference. Between bed 1 and the supporting structure 10 there is a space 14 for the rejected portion, the space having connection unit 15 for removing the rejected portion from the pressurized screening device.
• The plurality of baffle blades 12 consists of a number of baffle blades 13 whose inner edge 16 within its middle is attached to the body of the plurality of baffle blades 12a, the body, in turn, being attached to the shaft assembly 8 (figure 1) or to the supporting structure 10 and to the radial supporting arms 20 (figure 2). The baffle blades 13 are directed radially outward towards the screenplate 7. The outer edge 17 of the baffle blades 13 is essentially parallel with the screenplate. The edge, that is nearest to the feed connection unit 4, of the baffle blades is beveled outward from the centre line (point 18, figures 1 and 2).
• The embodiment shown in figure 1 is so designed that devices 11 and the plurality of baffle blades 12 are provided with separate driving mechanisms, i.e. belt transmission K1 for devices 11 and belt transmission K2 for the plurality of baffle blades 12 whereupon preferably a common driving mechanism like a combination of an electric motor, clutch and cone belt pulley drives the two concentric shafts indicated by number 8 in figure 1 the number referring generally to the shaft system. It is obvious that a construction performing the corresponding functions can be accomplished also by means of a clutch mounted at position V in the embodiment shown in figure 1, whereby only one driving mechanism along with the shaft is needed. In this case the plurality of baffle blades 12 can, in the first place, be locked non-rotating relative to the pressurized screening device whereupon the tangential velocity of the fibre pulp is nearly zero when it leaves the outer edges 17 of baffle blades 13 of the plurality 12. This can be accomplished, if necessary, so that the part of the shaft assembly 8 which drives the plurality of baffle blades 12 is released from the driving mechanism and is locked relative to the pressurized screening device or so that the gear (point V, figure 1) which transmits the rotational force to the devices 11 is detached from the shaft assembly 8 and the plurality of baffle blades 12 is locked in place so that it is immobile relative to the pressurized screening device. By the above arrangement the tangential velocity of the fibre pulp can be controlled while the pressurized screening device is in operation, in other words it is a so-called controllable pressurized screening device. Rotation of the plurality of baffle blades 12 can be slowed down or speeded up and then, naturally, the tangential velocity of the fibre pulp is affected as it leaves the plurality of baffle blades. Naturally, the overall screening performance can also be affected simultaneously or by separate measures by changing the rotational speed of devices 11. These measures can be accomplished, for instance, by means of an outside brake mechanism or a drive mechanism.
• In the embodiment of figure 2 the plurality of baffle blades 12 is supported to the supporting structure 10 on the one hand and to the supporting ring 19 on the radial supporting arms 20 on the other hand, the supporting arm being placed on the upper edge of the devices 11 for treating screenplate 7. The inner edges 16 of the baffle blades can, as in this case, be essentially parallel with the outer edges. In the embodiment of figure 2 the radial dimension of the baffle blades of the plurality 12 can be changed by attaching the baffle blades to different radial positions in the supporting structure 10 and by means of fastening elements 21 like guide bars provided with holes for bolt joints. In the embodiment of figure 2 the velocity ratio is arranged fixed since the plurality of baffle blades 12 rotates along with the device 11 with a rotational velocity determined by the driving mechanism K like the cone belt transmission of the shaft assembly 8. The driving mechanism K is driven by an aggregate like the combination of an electric motor and a cone belt pulley (not shown).
• This solution can be further clarified by the following calculation: Denoting

D₁

= the cross-sectional diameter of the screenplate

D₂

= diameter of the circle defined by the outer edges of the plurality of baffle blades 12 and

D₃

= diameter of the circle defined by the inner edges 16 of the plurality of baffle blades,

for instance
D₂ = 0,4 ...0,7 * D₁ and D₃ = approx. 0,2 ...0,3 * D₁, then as fibre pulp which is to be screened is introduced into the middle part of the plurality of baffle blades ( into the space with diameter D₃) and as the pulp passes by the baffle blades 13 of the plurality 12 the pulp flow gains a maximum rangential velocity, depending e.g. on losses,

${\text{V}}_{\text{t2}}\text{≈ N * π * D₂}$

, where


N

= speed of rotation (1/s) and

D₂

= diameter in meters.

• When this fibre pulp reaches screenplate 7 it has maintained the velocity V_t2 which is clearly less than the velocity of the devices 11 ( the rotor blades) for treating the screenplate
  
  ${\text{V}}_{\text{t1}}\text{= N * π * D₁}$

  

  , since D₂ is smaller than D₁.
  
• This shows clearly that by changing the ratio D₂/D₁ one can change the ratio of the tangential velocity of the fibre pulp to the velocity of the devices for treating the screenplate 7. As D₂ approaches D₁ one arrives at a situation where the tangential velocity of fibre pulp as it leaves the plurality of baffle blades 12 approaches the velocity of the devices for treating the screenplate 7.
• It is obvious that embodiments of figures 1 and 2 can be united into the same pressurized screening device within the alternative embodiments of the basic idea of the invention.

Claims (9)

1. Method of controlling a pressurised screening device wherein fibre pulp is fed into the pressurised screening device and classified by means of a cylindrically shaped perforated screenplate (7) into an accepted portion passing through said perforations of the screenplate (7) and into a rejected portion which is removed from the screenplate, wherein the perforated screenplate (7) is treated by means of a screenplate treating device (11) located within and moving relative to said screenplate, wherein the screenplate treating device (11) is designed to be rotated round the centre-line of the cross-section of the cylindrical form of the screenplate (7),
      characterized in that

   - fibre pulp is fed essentially parallel with the cylindrical axis centre line of the screenplate (7) into a baffle blade assembly (12) located within said screenplate and comprising two or more baffle blades (13) each having an outer edge (17), i.e. the edge that faces the screenplate, essentially parallel with said screenplate;

   - the baffle blade assembly (12) is caused to rotate essentially round said cylindrical axis; and in that

   - the speed of rotation of said baffle blade assembly and/or the radial position of the outer edge of said baffle blades (13) is controlled.
2. Method according to claim 1, characterized in that the plurality of baffle blades (12) is provided with locking means by which it can be made non-rotating.
3. Pressurized screening device comprising a cylindrically shaped perforated screenplate (7) placed inside a housing (2), a screenplate treating device (11) located within and moveable relative to said screenplate (7), means for generating a relative motion between the screenplate and said screenplate treating device (11), and piping means (4, 6, 15) firstly for introducing fibre pulp into the pressurized screening device, secondly for removing the accepted portion of fiber pulp from the pressurized screening device and thirdly for removing the rejected portion of fiber pulp from the pressurized screening device;
      characterized in that said pressurized screening device further comprises a baffle blade assembly (12) located within said screenplate and comprised of two or more baffle blades (13) each having an outer edge (17), i.e. the edge that faces the screenplate, essentially parallel with said screenplate, said baffle blade assemply being rotational essentially round an axis parallel with the centre line of the screenplate (7); and in that
      said piping means (4, 6, 15) feed the pulp into said pressurized screening device essentially parallel with the centre-line of the screenplate towards the plurality of baffle blades.
4. Pressurized screening device according to claim 3 characterized in that the plurality of baffle blades (12) and the screenplate treating device (11) are provided with separate driving mechanisms (K1, K2), to which both are separately connected.
5. Pressurized screening device according to claim 3 characterized in that the plurality of baffle blades (12) is attached to the driving mechanisms (K) common to the screenplate treating device (11).
6. Pressurized screening device according to claim 5 characterized in that the plurality of baffle blades (12) is attached to the supporting structure (10) of the screenplate treating device (11) and/or to an element (20) in connection with the supporting structure.
7. Pressurized screening device according to claim 5 and 6 characterized in that the plurality of baffle blades (12) is designed to be movable and lockable relative to the supporting structure (10) and/or to an element (20) in connection with the screenplate treating device (11).
8. Pressurized screening device according to claims 5-7 characterized in that the housing 10 and the radial supporting arms (20) of a supporting ring (19) or the like in connection with the screenplate treating device (11) are provided with a plurality of guide bars (21) or the like for locking the baffle blades (13) in radial direction at an optional position.
9. Pressurized screening device according to claims 3 and 5 characterized in that a gearing is placed between the body (12a) of the plurality of baffle blades (12) and the driving mechanism (K) common to the plurality of baffle blades (12) and the screenplate treating device (11).

Images